Visteon Automotive Systems - Windshield De-icing Model

The de-icing of an automotive windshield is simulated using FLUENT (from Fluent Inc.) in this example. A finite layer of ice on the windshield is gradually melted in the transient analysis that includes the phase change of ice from solid to liquid. The FLUENT computational fluid dynamics (CFD) predictions are in excellent agreement with the melting patterns observed in a test vehicle.

The task of designing a windshield defroster is a difficult one. The defroster must adhere to government regulations regarding the time to clear a minimum specified area of the windshield. Previously, the process involved design, then construction of a trial defroster, followed by a testing program. Based on test results, adjustments to the initial design were made and the process repeated. At Visteon Automotive Systems, engineers have recently used CFD to augment this process. With a novel approach that uses a phase change model in FLUENT, they have been able to successfully predict the melt pattern that is observed experimentally. This success has paved the way to a more rapid turnaround time in the design phase of these systems.

The de-icing simulation performed at Visteon uses a steady-state flow field that emanates from grill work at the base of the inside of the windshield. The temperature of the air steadily increases with time and reaches steady-state based on a heater warm-up curve that is representative of the defroster output during startup and normal operation. The windshield glass is modeled as a conducting wall, and a finite layer of solid ice is present on the outside of the windshield at the start of the calculation. The phase change model implemented in FLUENT through user defined functions makes use of an enthalpy-porosity method to track the fraction of solid (and liquid) in each control volume on the outside of the windshield. When the solid fraction reaches zero, the ice has melted and the windshield is assumed clear in that region.

To validate the model, the FLUENT predictions for a solid fraction of zero on the outside of the windshield can be compared to test results for the defrost patterns at different times. The defrost pattern after 5 minutes of the CFD calculation (blue and white) is compared to test results after 5minutes of defroster operation (black lines). Good agreement is seen.Note that the FLUENT results show not only the regions where complete melting has occurred, but regions where the ice layer is beginning to thin and where complete melting will subsequently occur. After 10 minutes of operation, the agreement between experiment (black line) and the FLUENT prediction is excellent. This agreement gives support for the phase change model to be used for this type of complex analysis.

Fluent Inc. - Automotive Division, 220 E. Huron, Suite 470, Ann Arbor, MI 48104 . Tel: 734-213-6821; Fax: 734-213-0147.